The present invention relates to vehicle transmissions, especially for heavy, on- and off-road vehicles, and more particularly to dual- and multi-clutch transmissions according to the preamble of the independent claim.
Dual clutch transmissions are a cross-breed between conventional stepped transmissions, with power interruption at gear shifts, and powershifting, without power interruption, planetary transmissions. In principle, a dual clutch transmission has two input shafts, each connectable to a friction clutch and to the output of the engine. Functionally, this is equivalent to having two conventional transmissions in parallel, i.e., two parallel sub-transmissions, and using one at a time for power transfer. The sub-transmission that is not used, idling, for the time being, can have a gear engaged and prepared, preselected, for a subsequent shift. This shift is carried out by simultaneously disengaging the friction clutch of the previously used sub-transmission and engaging the friction clutch of the previously idling sub-transmission.
When properly designed, dual clutch transmissions have the potential of providing powershifts at a reasonable production cost and low power losses. This is due to the fact that the rotating parts, i.e., gearwheels, shafts and tooth clutches, are similar to those in conventional stepped transmissions. This, furthermore, enables the use of the same production equipment. So, it makes sense to produce dual clutch transmissions in the same facilities as used for conventional stepped transmissions.
Dual clutch transmissions often have two separate countershafts, one connected to each input shaft. One example is U.S. Pat. No. 4,876,907. These countershafts make the transmission considerably wider than a conventional stepped transmission. That may lead to difficulties in installing the transmission into the vehicle. However, some dual clutch transmission designs have only one countershaft, e.g., as in DE923402 and DE3131156A1. Loose gearwheels are arranged rotatably thereon and can be rota-tionally connected to each other and to the countershaft by mechanical tooth clutches. In a way, this could be seen as if the second countershaft is arranged coaxial to the first one. The result is a very compact powershiftable dual clutch transmission that is not wider than a corresponding conventional stepped transmission.
Normally, in a dual clutch transmission, gears are preselected in the presently idling sub-transmission by engaging and disengaging tooth clutches. For a smooth and durable operation, this requires that the parts to be engaged by a tooth clutch are synchronized, i.e., that they have fairly equal rotational speed. If not, the clutch teeth would clash, resulting in worn, or broken, teeth and noise. So, different kinds of devices and arrangements are used for synchronizing parts to be engaged. This is also the case for conventional stepped transmissions that have a power interruption at each gear shift. There is, however, one important difference. At a power interruption, the engine speed can be controlled in order to synchronize parts to be engaged. This is a procedure used in automatic mechanically engaged transmissions (AMTS) that are common in heavy trucks and buses. In a dual clutch transmission without power interruption, this is not possible. Instead, some synchronizing devices are required. A straightforward solution is to have every tooth clutch in the transmission designed as synchronizers, i.e., be equipped with synchronizing clutch elements, e.g., as in US2008/0188342A1. That would imply high costs and power losses, though.
Basically, only two synchronizing devices are required; one that can make the speed of the first sub-transmission larger than that of the second sub-transmission, and one that can make the speed of the first sub-transmission less than that of the second. That will work when the first sub-transmission is idling and the second is active, as well as when the first sub-transmission is active and the second is idling. This can be referred to as a central synchronizing unit. A lucid example thereof is shown in GB2110324A.
A central synchronizing unit in a dual clutch transmission can be of a very simple design. This normally requires, however, that shifts without power interruption are performed between consecutive gears, only. Furthermore, the speed ratio steps between consecutive gears should be fairly equal. For heavy on- and off road vehicles, in combination with a range section, e.g., as in US2008/0188342A1, this is plausible. In U.S. Pat. No. 4,876,907 a central synchronizing unit 30 makes use of a gearwheel 13 that is used for power transfer. Then, only three additional gearwheels, 32, 33 and 39 are required for the synchronizing function. These gearwheels only need to carry the loads at synchronizing, and can be considerably narrower than the power transferring gearwheels. The central synchronizing unit can, hence, be made very compact, especially in axial extension, and cost-effective.
Tooth clutches that do not have any synchronizing clutch elements can be referred to as unsynchronized tooth clutches. Special care is required for transmissions with un-synchronized tooth clutches. A case with operating engine and the vehicle at standstill can be considered. Shafts and gearwheels that are drivingly connected to the output of the transmission, and hence to the driven wheels, will not rotate. Other parts of the transmission are drivingly engaged to the engine, e.g., for driving a power take-off unit, and rotate. Now, if the vehicle is to be driven, the engine and the power take-off unit need to be disengaged, and a start-off gear needs to be engaged in the transmission. If the tooth clutch that corresponds to the start-off gear is unsynchronized, it cannot be engaged until the speed of the rotating parts is low enough. With no special means therefore, that may take several seconds, which hardly is acceptable. Thence, transmissions with unsynchronized tooth clutches often have a shaft brake that can selectably reduce the speed of rotating parts. Examples can be seen in U.S. Pat. No. 3,309,934, DE19652916A1, U.S. Pat. No. 5,988,344, US2003/0168300A1, WO2004/069621A1, U.S. Pat. No. 7,000,748B2, SE527267C2, WO2008/094122A1, WO2008/105728A1, and for a dual clutch transmission, DE3739898A1. A shaft brake normally acts on a shaft or on a gearwheel drivingly connected therewith.
In a dual clutch transmission, two shaft brakes could be used, one for each sub-transmission, in order to reduce the speed of rotating parts when the vehicle is at standstill. That would increase the costs, though.
A constraint with a central synchronizing unit is that it only enables shifts between consecutive gears. It would be of advantage if some multi-step gear shifts could be carried out, e.g., at rapid acceleration from low vehicle speed in steep downhill conditions.
Thus, it is desirable to present an improved multi-clutch transmission, where means are provided to facilitate i) engagement of gears when the vehicle is at standstill, and ii) multi-step powershifts, as well as being iii) compact, iv) powerful, and v) cost-efficient, and vi) having low power losses when not actuated. This is achieved by a multi-clutch transmission as discussed in the introduction, the characteristics of which are defined by claim 1.
The device according to an aspect of the present invention is a multi-clutch transmission for a motor vehicle with at least one prime mover,
said multi-clutch transmission comprising frictional clutches drivingly connected to said prime mover, an output shaft, and a main transmission that comprises input shafts connected to said frictional clutches, a countershaft parallel to at least one of said input shafts, gearwheels and tooth clutches,
where a first of said frictional clutches and a first subset of said tooth clutches are arranged to be selectively engaged in order to transfer power in a first set of gears between said prime mover and said output shaft, and where a second of said frictional clutches and a second subset of said tooth clutches are arranged to be selectively engaged in order to transfer power in a second set of gears between said prime mover and said output shaft,
where said main transmission is arranged to, while transferring power in one of said second set of gears and having said first frictional clutch disengaged, being capable of having one of said first set of gears preselected, and of changing to have another of said first set of gears preselected,
where said multi-clutch transmission is arranged to be capable of shifting from one of said second set of gears to one preselected of said first set of gears without interruption of the power transfer between said prime mover and said output shaft,
characterized in that a shaft brake unit is arranged to be selectively activated in order to exert torque that tends to reduce the rotational speed of one of said input shafts, and a central synchronizing unit is arranged to be selectively activated in order to make a first of said input shafts to rotate faster or slower than a second of said input shafts.
In another embodiment according to the invention said shaft brake unit and said central synchronizing unit are further arranged to be simultaneously activated in order to reduce the speeds of rotating parts in said main transmission prior to the engagement of tooth clutches when said output shaft is not rotating.
In a further embodiment according to the invention said shaft brake unit is a countershaft brake unit arranged to brake said countershaft and one of said input shafts via two of said gearwheels, where one of said gearwheels is arranged rotationally fixed to said countershaft and the other of said gearwheels is arranged rotationally fixed to one of said input shafts and where said two gearwheels are in mesh.
In another embodiment according to the invention said shaft brake unit is an input shaft brake unit arranged to brake one of said input shafts directly by braking interaction between a first element rotationally fixed to or integrated with said input shaft and a second element fixed to or integrated with a multi-clutch transmission housing.
In yet another embodiment according to the invention said input shaft brake unit is arranged to be used when performing a multi-step power shift.
In a further embodiment according to the invention said main transmission is arranged to perform said multi-step power shift from a low gear, which is a gear in said second set of gears, to a higher gear, which is a gear in said first set of gears of said input shaft, which said input shaft brake is arranged to brake, and in order to be able to engage said higher gear said input shaft brake is arranged to decrease the rotational speed of said input shaft to a level that corresponds to the gear that is to be preselected of said first set of gears.
In a further embodiment of the above mentioned embodiment said input shaft brake is arranged to decrease rotational speed of said input shaft to that of said main shaft, upon which said input shaft is arranged to be drivingly connected directly to said main shaft by one of said tooth clutches in said first set of gears.
In a further embodiment of one of the above four latest mentioned embodiments said first element is an input shaft synchronizing gearwheel and said second element is a brake pad.
In a further embodiment of the above first mentioned embodiment said shaft brake unit is arranged to be activated prior to the preselection of a third of said first set of gears while transferring power in a first of said second set of gears in order to facilitate a multi-step power shift.
In a further embodiment of the above mentioned embodiment there is a second of said first set of gears whose speed ratio is between the speed ratios of said third of said first set of gears and said first of said second set of gears, and which second of said first set of gears is skipped when said multi-step power shift is performed.
In a further embodiment of the above mentioned embodiment one of said gearwheels transfers power in one of said first set of gears and in one of said second set of gears, and zero power is transferred via said one of said gearwheels in said third of said first set of gears and in said first of said second set of gears.
In a further embodiment of one of the above three latest mentioned embodiments two of said gearwheels are in mesh and transfer power in a low of said first set of gears and in a low of said second set of gears, where the speed reduction ratio of said low of said first set of gears is larger than that of said low of said second set of gears.
In a further embodiment according to the invention said central synchronizing unit is arranged to be selectively activated in order to make a first of said input shafts to rotate corresponding to less than two gear steps faster or slower than a second of said input shafts.
In a further embodiment according to the invention said multi-clutch transmission is a dual clutch transmission.
Further advantageous embodiments of the invention emerge from the following.